1. Field of the Invention
The present invention relates generally to motor driven compressors and, more particularly, to a compressor that is utilized as a vacuum pump and driven by a variable speed motor. Even more particularly, the present invention is directed to a screw compressor used as a vacuum pump and driven by a variable reluctance motor, wherein the variable reluctance motor is connected in fluid communication with the inlet port of the compressor so that the internal portion of the variable reluctance motor can be maintained at a pressure below atmospheric pressure.
2. Description of the Prior Art
It is well known to drive a compressor with a motor and equally well known to drive a screw compressor with an electric motor. It is also well known to those skilled in the art to utilize a screw compressor for the purpose of maintaining a vacuum in a system. When utilized in this manner, the compressor is generally referred to as a vacuum pump.
U.S. Pat. No. 3,790,309, which issued to Volz on Feb. 5, 1974, describes a unitary pump and motor assembly with a common drive shaft. Although the pump is not a screw pump in the strictest sense of this terminology, it is driven by the motor and a secondary flow of liquid from the compressor is forced to flow through the motor as a result of the presence of helical passages in the rotor.
U.S. Pat. No. 3,740,630, which issued to Jarret et al on Jun. 19, 1973, describes a variable reluctance electric motor. Although the variable reluctance electric motor described in this patent is not attached to a pump, the cross sectional views shown in the patent illustrate the different geometric configurations disposed in the air gap of the motor as compared to the air gap of a conventional electric motor such as that described in U.S. Pat. No. 3,790,309 discussed above.
German Patent 207,956 describes a cooling system for a screw compressor assembly. To reduce the temperature of the motor which drives the screw compressor, a small portion of the output from the compressor is directed to an inlet of the motor and caused to flow through the motor prior to returning to an additional inlet of the screw compressor. From a description of the invention, it appears that the fluid flowing into the motor is a liquid. Since the fluid is flowing to the motor bypasses the expansion valve 5 and evaporator 4 of the system described in this patent, it further appears that the liquid entering the motor 2 is expected to be caused to evaporate by the heat of the motor and the relevant pressure changes prior to entering the inlet port of the compressor as a gas.
As a result of recent development in the field of variable reluctance motors and their electronic control systems, it has become advantageous to use variable reluctance motors to drive screw compressors. One of the significant advantages of using a variable reluctance motor to drive a screw compressor is the fact that the variable reluctance motor permits the motor and compressor to be operated at a virtually infinite variety of speeds within the capacity of the motor. This, in turn, permits the compressor to be controlled in a manner which responds advantageously to changes and demands and pressures within the system.
Notwithstanding these significant advantages, the use of variable reluctance motors also create certain disadvantages which must be addressed. First, the shape of the rotor in a variable reluctance motor is such that it does not usually have a smooth outer cylindrical surface. Instead, it has very prominent poles that extend radially from the central axis of the rotor. These poles create severe irregularities in the rotor shape and these irregularities exacerbate the air resistance and noise problems that occur when one member of a dynamoelectric machine rotates in close proximity to another member. In other words, when the irregularly shaped motor rotates within the stator of a variable reluctance motor, the air resistance encountered by the rotor is more severe than that encountered by a rotor of a conventional electric motor with a smooth outer cylindrical surface. Not only does the rotor encounter more severe air resistance but, in addition, the movement of the rotor through the air creates a significant source of noise within the region of the air gap of the variable reluctance motor. These problems are further exacerbated by the fact that variable reluctance motors are typically operated at speeds much higher than those of conventional electric motors.
To improve the motor driven compressor system which utilizes a variable reluctance motor, some means must be provided to reduce the air resistance and noise which are incumbent with the use of a variable reluctance rotor because of its irregular rotor shape and high speed of operation. The present invention is directed to the solution of those problems.